Usually damping devices of the type cited above can find use in various industrial aplications, for example, they are disposed between an oscillating mass constituted by the engine of motor-vehicle and the body-chassis of the vehicle itself.
Generally speaking, according to some solutions, hydraulic damping devices can comprise a rigid container, an elastomeric membrane that forms the lid of the container, an elastomeric diaphragm which separates the space inside the container into two chambers, i.e., respectively upper and lower chambers, and a second membrane forming the base of the second chamber, with a low-viscosity liquid being disposed in both these two chambers.
The upper and lower chambers are hydraulically connected to one another by an annular conduit which extends over a predetermined arc, that is provided with two extremities, one present in the upper chamber and the other in the lower chamber.
In a device of this type, when the mass oscillates with a low-frequency and small a amplitude, for example, with a frequency between 100 and 200 Hz. and an amplitude between 0.1 and 0.2 mm, practically speaking, no shifting of the liquid occurs, between the first and second chambers, because due to the speed with which the oscillations take place, this liquid is unable to traverse the annular conduit which is selected to have apt dimensions.
The pressure variations and volume variations in the liquid are absorbed by the elastomeric deformation of the membrane that forms the lid and by the shifting of the diaphragm between two extreme positions.
When the mass oscillates, with a low-frequency and a great amplitude, for example, with frequency values between 5 and 30 Hz., and amplitudes between 1 and 5 mm, it so happens that the diaphragm is brought up against the opposite fixed surfaces joined to the container walls and the liquid is forced to pass through the annular conduit with the result that energy is dissipated due to viscous damping. In practice, in the cited devices, the geometrical dimensions of the annular conduit, e.g., the length and value of the transverse section, are chosen so as to allow the passage of the liquid between the first and the second chamber, then vice-versa, at a predetermined value of low-frequency and great amplitude, with a corresponding damping of the relative oscillation.
The known device, generally comprise an annular conduit which is constructed by approaching two rigid parts together.
Moreover, the two rigid parts, which are spaced apart from the conduit, form special pressing means for the outermost annular edge of the elastomeric diaphragm.
In these types of devices, due to the high pressure with which the liquid is forced into the annular conduit, it could happen that liquid is drawn through the contacting surfaces of the two halves of the conduit itself.
As can be readily appreciated, any such drawing of liquid is not acceptable, since it causes the viscous damping to be irregular at the predetermined frequencies.
Moreover, in the cited devices, the additional pressure of the liquid tends to draw apart the two rigid halves forming the conduit, and hence, to diminish the pressure of the pressing means upon the diaphragm.
Therefore, the known devices possess the drawbacks of (1) no reliable fluid-tight sealing of the annular conduit and (2) no satisfactory locking action at the edges of the elastomeric diaphragm.
Moreover, even if these cited drawbacks could be overcome with a simple construction that will prove to be reliable with the passing of time, it has also been verified that the known devices present an excessive rigidity when faced with high-frequencies, i.e., the known devices tend to transmit, with values substantially unaltered or in any event with high values, the oscillations of high frequency and small amplitude, to the vehicle body, and hence also to passengers inside the vehicle.